A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. The transistor is used in a wide range of electronic devices such as an integrated circuit (IC) and a display device. As semiconductor materials which can be used for the transistors, silicon-based semiconductors have been widely used, but oxide semiconductors have been attracting attention as alternative materials.
For example, a technique for forming a transistor using zinc oxide or an In—Ga—Zn-based oxide semiconductor as an oxide semiconductor is disclosed (see Patent Documents 1 and 2).
Patent Document 3 discloses that an imaging device in which a transistor including an oxide semiconductor and having an extremely low off-state current is used in part of a pixel circuit and a transistor including silicon with which a complementary metal oxide semiconductor (CMOS) circuit can be formed is used in a peripheral circuit.
Patent Document 4 discloses an imaging device in which a transistor including silicon, a transistor including an oxide semiconductor, and a photodiode including a crystalline silicon layer are stacked.